Additive manufacturing (AM), colloquially referred to as 3D printing, is the process of building a three-dimensional object by adding the necessary materials layer by layer. The term “3D printing” encompasses a number of additive manufacturing techniques including fused filament fabrication (FFF), select laser sintering (SLS), and stereolithography (SLA). These methods are described in more detail in the following U.S. patents and patent publications, all hereby incorporated by reference in their entirety: U.S. Pat. Nos. 4,575,330, 5,597,520, 7,959,847, 5,247,180, 8,252,223, U.S. Patent Pub. No. 2014/0268604, U.S. Pat. Nos. 4,078,229, 5,121,329, 6,103,176, 5,637,175, and 8,827,684.
The FFF process is based on a thermoplastic filament fed through a heated extrusion nozzle at a controlled rate and deposited as a continuous feed of molten plastic at discrete locations on a build plate. The FFF process uses a three axis platform typically actuated with stepper motors and an extruder assembly composed of a plastic filament driver and a hot end. Careful control of printing parameters allows complex geometries to be constructed.
The two major part components can be manufactured using the FFF process are shells and infill. Shells are the outermost layers that form the part's geometry while infill is the interior support for the shells. High infill percentages help to increase strength of printed parts by providing more structural support. However, there is a tradeoff between build time and part strength as dictated by infill. The FFF process uses thermoplastics such as polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), high impact polystyrene (HIPS), and various polyamides (Nylons).
SLA uses photo-curable resins and a light source such as a laser or Digital Light Processing™ (DLP) (Texas Instruments Corp.) projector to cure each successive layer in a part. Liquid photopolymer resin is selectively bombarded with focused light to cure the photopolymer to a solid state. Typically, SLA uses a vat of photopolymer with a height-adjustable build plate that enables layer-based manufacturing. UV light is commonly used to cure the photopolymer in a layer pattern specified by software. SLA uses a laser to trace each layer while a DLP SLA machine uses a digital projector to cure an entire layer at once, decreasing build time. The SLA AM method also typically results in higher resolution parts than FFF. Material properties (e.g. percent elongation, tensile strength, brittleness, etc.) vary greatly in SLA due to the large number of photopolymer blends available and inconsistent material properties among blends. Disadvantages of SLA include UV stability, brittleness, warping, internal stresses and cure imperfections such as air pockets.
Composites combine multiple materials to achieve desired material properties not feasible through the use of one bulk material. Typically, composites encompass a support matrix, such as an epoxy, reinforced with a phase material such as carbon fiber, fiberglass, or metal fiber. Composite parts exhibit different failure modes than single-material parts. Composite failure modes can be classified as a matrix failure, where the bonding between the matrix and fiber fails, or a phase failure, where the fibers themselves fail. These failure modes are more akin to AM part failures where failure typically occurs at layer boundaries. While multi-material approaches are used in AM with features like dissolvable material and color printing, multi-material processes can also be used to achieve desirable mechanical properties as seen in composites. Current composite FFF technologies have been classified into three categories including reinforced filaments, doped filaments, and ultrasonic/thermal embedding.
AM has gained a tremendous amount of interest among engineers, scientists, and the maker community over the last decade. Several AM processes such as FFF and SLA have recently become affordable for a broad range of applications and have spurred the rapid adoption of AM in many different industries such as aerospace, automotive, medical, and advanced manufacturing. Driven by several factors including rapid low production volume part runs, specialized tooling requirement elimination, and reduction of penalization for part complexity, AM is changing the way low production volume parts are manufactured. However, the benefits of AM come with several challenges and opportunities for improvement. AM contains a whole new set of challenges typically absent in subtractive manufacturing (SM) processes such as delamination failures, large internal stresses, and porosity. FFF and SLA, two most widely used AM methods for plastics, face significant issues for production run part adoption. One such issue is the material properties of the printed parts. Layer-based manufactured parts have a unique set of failure modes that are not generally found in SM parts and are more akin to failures found in composite materials. Furthermore, mechanical properties of AM parts produced by economical commercial options are inconsistent across production runs due to process variations (e.g. extruder temperature, ambient temperature, bed temperature, extrusion rate, print speed, etc.).
Various methods to strengthen inter-layer bonding are available such as increasing extrusion nozzle temperature, decreasing layer height, elevating build volume temperature, and additional techniques primarily focused on increasing polymer chain interactions between layers. The tensile strength gains are still modest and do not equal or surpass bulk material properties. While polymer AM processes such as FDM and SLA can produce quality non-load bearing parts, there is a need for an alternative capable of producing economical load bearing-parts.
FFF and SLA have additional issues. These methods are notoriously slow at producing objects and have limited compatible materials, limiting the commercial viability of AM. Accordingly, there is also a need for a process and device which improves the print times and can be used with a wide range of materials to improve the range of achievable part properties.